1. Field of the Invention
The present invention relates to an opto-electric hybrid board including an optical waveguide and an electric circuit board which are stacked together, and a method of manufacturing the same.
2. Description of the Related Art
With the increase in the amount of transmission information, optical interconnection in addition to electrical interconnection has been used in recent electronic devices and the like. As an example of such a technique, an opto-electric hybrid board has been disclosed in Japanese Laid-open Patent Application Publication No. 2010-164655. As shown in FIG. 10, this opto-electric hybrid board includes: an electric circuit board E0 including an insulative substrate 51 made of polyimide and the like, and electrical interconnect lines 52 formed on the front surface of the insulative substrate 51; and an optical waveguide (optical interconnect lines) W0 (including an under cladding layer 56, cores 57 and an over cladding layer 58) made of epoxy resin and the like and stacked on the back surface (a surface opposite from the surface with the electrical interconnect lines 52 formed thereon) of the insulative substrate 51.
In the aforementioned opto-electric hybrid board, however, the insulative substrate 51 (polyimide resin and the like) and the optical waveguide W0 (epoxy resin and the like) are in contact with each other. A difference in coefficient of linear expansion between the insulative substrate 51 and the optical waveguide W0 causes stresses and slight bending in the optical waveguide W0 at ambient temperature, resulting in increased light propagation losses in the optical waveguide W0.
Another opto-electric hybrid board, on the other hand, has been disclosed in Japanese Laid-open Patent Application Publication No. 2009-265342. As shown in FIG. 11, this opto-electric hybrid board includes a stainless steel layer M0 provided entirely between the insulative substrate 51 and the optical waveguide W0. In this opto-electric hybrid board, the stainless steel layer M0 serves as a reinforcement to prevent stresses and slight bending in the optical waveguide W0, thereby suppressing the increase in light propagation losses.
The opto-electric hybrid board is required to suppress the increase in light propagation losses, as stated above. In addition to this, the opto-electric hybrid board is required to have flexibility in some cases. However, the stainless steel layer M0 acts as an impediment to flexibility when provided entirely as mentioned above (with reference to FIG. 11) for the purpose of decreasing the light propagation losses.